docs: def _get_SRF_tau(self, imt_per):
          """
          Table 6 and equation 19 of 2013 report.
          """
          if imt_per < 1:
              srf = 0.87
          elif 1 <= imt_per < 5:
              srf = self._interp_function(0.58, 0.87, 5, 1, imt_per)
          elif 5 <= imt_per <= 10:
              srf = 0.58
          else:
              srf = 1
  
          return srf